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Salami-like Electrospun Si Nanoparticle-ITO Composite Nanofibers with Internal Conductive Pathways for use as Anodes for Li-Ion Batteries Daehee Lee,† Bokyung Kim,† Joosun Kim,*,‡ Sunho Jeong,§ Guozhong Cao,¶ and Jooho Moon*,† †
Department of Materials Sciene and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea High-Temperature Energy Materials Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 136-791, Republic of Korea § Advanced Materials Division, Korea Research Institute of Chemical Technology, 19 Sinseongno, Yuseong-gu, Daejeon 305-600, Republic of Korea ¶ Department of Materials Science and Engineering, University of Washington, 302M Roberts Hall, Box 352120, Seattle, Washington 98195-2120, United States ‡
S Supporting Information *
ABSTRACT: We report novel salami-like core−sheath composites consisting of Si nanoparticle assemblies coated with indium tin oxide (ITO) sheath layers that are synthesized via coelectrospinning. Core− sheath structured Si nanoparticles (NPs) in static ITO allow robust microstructures to accommodate for mechanical stress induced by the repeated cyclical volume changes of Si NPs. Conductive ITO sheaths can provide bulk conduction paths for electrons. Distinct Si NP-based core structures, in which the ITO phase coexists uniformly with electrochemically active Si NPs, are capable of facilitating rapid charge transfer as well. These engineered composites enabled the production of highperformance anodes with an excellent capacity retention of 95.5% (677 and 1523 mAh g−1, which are based on the total weight of Si-ITO fibers and Si NPs only, respectively), and an outstanding rate capability with a retention of 75.3% from 1 to 12 C. The cycling performance and rate capability of core−sheath-structured Si NP-ITO are characterized in terms of charge-transfer kinetics. KEYWORDS: lithium ion battery, silicon anodes, electrospinning, rigid conductive sheath, high charging rate capability
1. INTRODUCTION
contraction results in SEI layers that are routinely broken, exposing the fresh silicon surface to the electrolyte and resulting in gradual SEI growth. Consumption of lithium ions during SEI layer formation, the electrically insulating nature of the SEI layer itself, and the long diffusion distance necessary for lithium to diffuse through the thick SEI layer all contribute to drastic, irreversible capacity fading during cycling. Nanostructuring of Si in the form of nanowires, nanotubes, and hollow/porous particles can address the challenge of pulverization with significant improvements in electrochemical cycling, because the nanosized Si is capable of relaxing mechanical strain.7,8 Despite the success achieved with diverse nanostructured Si electrodes, complicated synthesis processes such as chemical vapor deposition (CVD) and/or templating/ etching processes, which are uneconomical and difficult to scale up, were used in previous studies. In contrast, the use of Si
Lithium ion batteries (LIBs) are an attractive energy storage system for a wide variety of applications including smart phones, laptop computers, and hybrid electric vehicles.1,2 However, advanced LIB technology still requires electrode improvement in terms of energy capacity, cycling stability, and charging rate capability. Metallic anodes, such as Si, Ge, and Sn, are promising alternatives to the conventional anodic material, graphite, which has a low theoretical specific capacity of 372 mAh g−1.3−5 Among these materials, silicon is one of the most promising next generation anode materials because of its high theoretical specific capacity (>3500 mAh g−1), scale-up potential, and low cost.6,7 However, Si anodes in general suffer from large volume expansion/contraction during the lithium alloying−dealloying process, which limits their cycle life. Volume change-induced stress generates cracks and pulverizes silicon, causing the Si to lose its electrical connection to the current collectors. Significant changes in the volume of Si anodes are also responsible for the continuous growth of a solid electrolyte interphase (SEI). Repetitive volume expansion/ © 2015 American Chemical Society
Received: September 8, 2015 Accepted: November 24, 2015 Published: November 24, 2015 27234
DOI: 10.1021/acsami.5b08401 ACS Appl. Mater. Interfaces 2015, 7, 27234−27241
Research Article
ACS Applied Materials & Interfaces
2. EXPERIMENTAL SECTION
nanoparticles (Si NPs) enables cost-effective and reproducible fabrication of commercially viable Si anodes with high weight loading. Si-NP-based approaches have been employed with scalable, one-dimensional (1-D) nanofibers, where the Si nanoparticles are surrounded by electrospun carbon shells.9−12 Electrospinning is used extensively to fabricate and scale up 1D nanostructures with various morphologies (nanofibers, hollow fibers, and core−sheath fibers) using various materials including polymers, ceramics, metals, and carbons.13−15 Sibased 1-D nanostructures have exhibited promising capacity and cyclability; the use of hollow nanofibers addresses the issue of repetitive volume change during cycling. Specifically controlling both SEI formation and volume expansion through the adoption of a static oxide sheath layer can extend cycle life to thousands of cycles. Several oxides such as silica, alumina, and titania have been used for surface coating/cladding of either individual Si particles or nanotubes.16−18 It has been reported that a CVD-derived SiOx clamping layer (doublewalled Si−SiOx nanotubes) effectively prevented pulverization, and confined the volumetric expansion of Si to the inner void space.17 SEI formation was suppressed by a SiOx barrier layer, which prevented diffusion of the liquid electrolyte into the Si layer. This allowed for the formation of a stable, thin SEI layer only at the surface of the SiOx. This strategy improved the cyclability to 6,000 cycles; however, the outer insulating SiOx layer allows only a portion of the inner silicon to participate in the Li−Si reaction under the condition of high current density due to the limited conductivity of the SiOx shell layer. Recently, core−shell-structured Si NPs@TiO2−x/C mesoporous fiber was found to have enhanced rate capability using a conductive, static oxide TiO2−x shell. Although the conductive static shell aided the rate capability, the Si NPs@TiO2−x/C anodes exhibited relatively poor cyclability when capacity retention after 50 cycles was 90% of the initial capacity at 1 C.18 To address this, a new approach to materials and structural design is needed for the progression of fibrous composites as stable high-performance anodes. This can be achieved by considering the influence of conductive oxide shells on the stability and electrochemical reactions of active Si constituents in electrodes. Herein, we report a novel engineered 1-D composite material with remarkable capacity retention and high rate capability. We prepare the composite anodes of Si NP-indium tin oxide (ITO) heterogeneous inorganic fibers with a salami-like morphological structure, through a one-pot, facile, coelectrospinning synthesis. ITO is a well-known transparent conducting oxide that has been extensively utilized in the electronic and display industries. It is utilized here to illustrate the concept of conductive-oxidebased composite electrodes prior to the testing other economic conducting oxides. The uniquely separated inorganic structure accommodates for the mechanical stress induced by the repetitive volume changes of the Si NPs during cycling. The ITO phase provides a conduction pathway and a sponge-like amorphous matrix after an irreversible partial conversion reaction, interconnecting the individually located Si nanoparticles, as well as accommodating for mechanical stress and thereby stabilizing the SEI formation. These well-engineered composites enable the production of high-performance anodes with excellent capacity retention and outstanding rate capability. The cycling performance and rate capability of salami-like structured Si NP-ITO composite electrodes are characterized in terms of charge transfer kinetics.
2.1. Synthesis of Composite Nanofibers. Si-ITO core−sheath nanofibers were prepared via coelectrospinning using a dual nozzle. To prepare the solution for the sheath, polyacrylonitrile (PAN, average Mw ≈ 150,000, Sigma-Aldrich) was dissolved in N,N-dimethylformamide (DMF, anhydrous 99.8%, Sigma-Aldrich) at a concentration of 8 wt % via stirring in a 20 mL vial at 60 °C for 4 h. Indium(III) nitrate hydrate (>99.99%, Sigma-Aldrich) and tin chloride (99.99%, SigmaAldrich) were added to the PAN solution at the composition of In/Sn = 95:5 mol %. The suspension for the core was prepared under the same conditions by dispersing Si nanoparticles (NPs) (
